Frequency translators for changing or translating r-f signals from one band of frequencies to another band of frequencies have heretofore been used extensively in the communications field. A more specific application for such frequency translators is for television signals in a CATV system to convert vhf signals from a cable to uhf signals capable of being applied to the television set uhf tuner.
A desirable criterion for a frequency translator for use in a CATV system includes the following:
(1) Low noise
(2) Low conversion loss
(3) High amplitude and phase conversion linearity
(4) Minimum spurious and harmonic modulation signal product generation
(5) Stable and accurate frequency conversion over a long time period without undue temperature variation effects
(6) Minimum oscillator signal emission at both input and output terminals.
Prior known frequency translators utilized for CATV systems have required relatively complex circuitry and as a result have been relatively expensive. A common approach of prior known frequency translators for CATV systems has been to utilize an active amplification stage such as a transistor in the input impedance matching and filtering network to compensate for relatively high conversion losses associated with a doubly balanced-type mixer and an output impedance matching or filter network. Moreover, the conventional approach in these frequency translators has been to utilize an active amplification stage usually including a transistor between the local oscillator and the mixer which functions as a unidirectional device to block or eliminate mixer-oscillator interaction. This blocking device has heretofore been a high reverse isolation buffer amplifier or a high reverse gyromagnetic isolator or circulator and functioned to minimize or eliminate unwanted signal products produced in the mixer from reflecting back to the oscillator which ultimately would alter the quality of the output signal from the mixer.
Prior known frequency translators for CATV systems in general have not been directed to circuit simplification and efficiency and have not recognized alternatives to the amplification of the input signal to make up for conversion losses or elimination of an active high reverse blocking device between the oscillator and the mixer and have not provided the most effective oscillators for this purpose.
Accordingly, the present invention is directed to achieving a novel and improved frequency translator for converting r-f television signals to a different frequency band in a highly efficient manner without sacrificing overall performance.
Another object of this invention is to provide a novel frequency translator capable of achieving the aforementioned desirable criteria in a highly efficient manner with a minimum of circuit components.
Still another object of this invention is to provide a novel and improved frequency translator for CATV systems characterized by the conjugate matching of the impedance between an improved oscillator and a singly balanced diode mixer to achieve the elimination of mixer-oscillator interaction, eliminate unwanted harmonic products in the output signal from the mixer, and reduce power losses.
Yet another object of this invention is to provide a novel and improved frequency translator for CATV systems that eliminates the need for an amplification stage for the incoming r-f television signal prior to its being applied to the mixer.
A further object of the present invention is to minimize this oscillator-mixer interaction by a simple means eliminating the need for a high reverse isolation buffer amplifier or a high reverse isolation (or circulator or any other) gyromagnetic isolator.